Class A Triode operation is well known to discriminating audiophiles because of its characteristic musicality and "warmth". This warmth of tonality can be ascribed to the avoidance of crossover distortion (because in a push-pull configuration neither device ever approaches cut-off) and to the soft or gradual onset of clip when dynamic levels exceed available undistorted power capabiity. Unfortunately these virtues of Triode Class A operation carry with them severe penalities of economy, efficiency and low power capability. Operating Class A means that less than half the supply power available can be converted to useful work, and that the devices themselves must be de-rated due to high zero-signal current draw. Connecting the screen grid of a Pentode to its plate, causing it to operate as a Triode, further reduces potential power gain by about half.
These penalties of price and power availability (not to mention heat) have prevented Class A Triode operation from ever gaining popularity in the Music Instrument field even more than in sound reproduction applications. This is particularly unfortunate for a couple of reasons: First, due to the extreme dynamic levels produced by the plucked string (e.g., electric guitar), amplifier output clip is virtually unavoidable. Second, musicians have done more than learn to live with amplifier distortion, they have incorporated it creatively into their musical expression and have become connoisseurs of different distortion characteristics. In the "heavy metal" idiom as well as the blues and popular milieu, elements of amplifier distortion are so strongly part of the guitar sound as to often comprise about 50 percent of actual sound output. Further, circuits to generate "desirable" distortion effects are well known and in use. These can be used to simulate output circuit distortion independent of output power level. If the history of professional choice were used as a yardstick of "desirable distortion" then clearly the circuit of Smith (U.S. Pat. No. 4,211,893) is a first choice. Here the option of distortion is available via remote switching and can be controlled to enhance solo or "lead" playing. It is interesting to note that Smith's circuit, which generates the distortion in the pre-amplifier section, uses Class A Triodes throughout.
The present invention, however, deals with output power amplification and the inherent distortion characteristics of different circuit configurations. Music is by nature a series of transient and fleeting events. The attack of any given musical note is of particular concern to the musician (as well as the listener) and much of a player's learned technique and expression revolves around the attack of the note. Such is clearly the case with all stringed instruments--including piano--as well as reed, brass and percussion instruments. Further, it can be shown that the manner in which an audio amplifier handles these transient attacks is the single most important factor that distinguishes an outstanding amplifier from one which is merely acceptable in both reproduction and live performance applications. The steady state distortion characteristic of most modern amplifiers is excellent: distortion of any type cannot be heard and competitive measurements have become pointless. Unfortunately though, there is no standard measurement of rating for distortion performance under actual dynamic conditions of use. To produce realistic sound levels in one's living room of a symphony or piano, without some clipping of the transient peaks would require an amplifier of at least 1000 watts and preferably much more.
So the demand for high power is obvious to the home user as well as the musician, yet the occurrance of amplifier distortion--clipping of peaks--remains an unavoidable fact of life to both. Whereas certain types of amplifier circuitry produce high power efficiently, a high sonic penalty is paid: their distortion is noticeable, harsh and disturbing. This is the typical "odd order" harmonic distortion which occurs in Class B type amplifiers and is the product of hard clip and sharp current transfer from push to pull, causing "crossover" or "notch" distortion. On the other hand, the Triode Class A circuit produces no crossover distortion and has a "soft" clip. The sound of amplifier distortion at clip and beyond is almost unnoticeable because clip does not occur suddenly, and when it does, it is characterized by the predominance of even order products which are actually harmonious musically (that is consonant, not dissonant) to the fundamental. But the penalty is power. Such a circuit is expensive and inefficient.
The well known amplifier Class AB offers some little improvement. Although the output devices operate Class A at low power, they become more and more Class B when driven harder and a somewhat harsh sounding distortion with an abrupt onset and visible crossover occurs at the crucial time: at clip. The power output and efficiency with Pentodes in an AB arrangement is fairly high however.